A number of continuous positive airway pressure devices are already known. Devices of this type currently being used in hospitals in first world countries generally require the supply of oxygen at high flow rates; since most modem hospitals have oxygen on tap and oxygen is available cheaply, a relatively wasteful use of oxygen is acceptable. However, in less developed countries or in out-of-hospital situations, (e.g. in ambulances, on battlefields, in emergency treatment stations) oxygen is available only in cylinders and a high rate of use of oxygen is unacceptable:—the cylinders simply are emptied too quickly.
It is therefore an object of the present invention to provide a continuous positive airway pressure device capable of operating at low flow rates of oxygen, whilst nevertheless reliably supplying the required air/oxygen mix to a patient.
In order to reduce the rate of the use of oxygen, it is necessary to use a reservoir built into the device. This is because the peak gas flow requirements of patient in acute respiratory distress can be anything from 60-120 liters per minute but practical engineering considerations mean that the gas flow rate through a portable continuous positive airway pressure device should be kept below 30 liters per minute. To make up the difference between gas demand and gas supply, portable devices must employ a storage system to store the gas while the patient is in between breaths or is breathing out. Any such storage system must be capable of a substantially constant pressure response, i.e. the pressure of the gas supply to the patient must not vary substantially whether the reservoir is full, part full or nearly empty.
A number of existing devices use a balloon reservoir made of elastic material or a reservoir shaped like an old-fashioned fire side bellows. Both of these designs have the drawback that the pressure of the gas supply to the patient varies considerably depending upon whether the balloon/bellows is nearly full, partly full or nearly empty. To overcome this drawback, a very much larger reservoir has to be used than is optimal for a portable device.
Some designs have been proposed to overcome this problem by providing a constant pressure response reservoir; see for example German Patent No.s DE 3712389 and EP 0744184.
German patent No DE 371-2389 discloses a reservoir of flexible material the upper surface of which is weighted either by a weight or by a weighted lever. However, the reservoir does not incorporate any type of stabilizing device to ensure that, as the bellows is compressed and expanded, the weighting on the reservoir is kept even.
EP 0744184 discloses a reservoir at least partly of elastic material, preformed so that the reservoir itself supplies compression to its contents. Since the pressure applied by a reservoir of this design would fluctuate markedly depending upon the volume of gas contained in the reservoir (i.e. according to the degree of expansion of the reservoir) a reservoir of this design could not provide a gas at a uniform or substantially uniform pressure.
A further drawback to both of the above described designs is that they relate to relatively large and complex devices which would be unsuitable for use outside a hospital environment.
Russian patent 459243 and German patent DE 410-7666 both disclose a reservoir in the form of simple bellows which is weighted by an upper moving plate. The moving plate can slide towards or away from a baseplate along fixed vertical guides. The guides are rigid and extend the full height of the expanded reservoir, resulting in a bulky apparatus which cannot be regarded as portable except in a hospital environment.
Indeed, a marked drawback of all known designs is the bulk of the equipment. Some of the devices, of course, are not designed to be portable at all, but even those devices which technically are portable are relatively large, fragile and easily damaged. A device which is classed as “portable” for a hospital environment may nevertheless be completely unsuited to being thrown into the back of a truck or into an aircraft, and may be too bulky to be stored conveniently for emergency use.